how fast does need to be an AI agent to be efficient?
My understanding of efficiency in this context is in regards to optimization of algorithms as opposed to hardware speed, which is more of a "brute force" component.  GPUs may be more energy efficient, but this is distinct from linear optimization of algorithms.

In terms of how much processor speed you need to tackle a given problem, that's in the realm of computational complexity theory and analysis of algorithms. 

Amount of GPUs, RAM and storage needed to tackle a given problem are purely a function of the complexity of the problem and the efficiency of the algorithms.